This invention relates broadly to the field of endodontics and particularly to endodontic files for use in preparing root canals to receive obturating material.
When a root canal of a living tooth becomes infected or abscessed, discomfort and, in many cases, sever pain begins. In the early days of dentistry the only solution was to pull the tooth. However, in recent years the science of endodontics has developed wherein practitioners can successfully remove the pulp material forming the nerve of the tooth that has become infected and, after careful preparation of the canal that contained the nerve material, refill the canal with an inert obturating material, such as gutta percha, permitting a patient to retain the tooth. In order to achieve a successful root canal restoration, the dental practitioner must carefully and, as completely as possible, remove the pulpal material making up the nerve system of the tooth to thereby prevent continued or future infection of remaining tissues. Further, the removal process must typically include shaping of the root canal so that it can be more effectively and successfully refilled and sealed with an inert obturating material to eliminate the possibility of further infection occurring within the cleaned and shaped root canal.
Cleaning and shaping the root canal to receive obturating material is achieved by the use of metal files that include cutting or scrapping working surfaces for removing tissue in the root canal. The working surfaces are typically formed by helical or spiral flutes cut therein. Generally more than one flute is provided, often axially spaced to create a land therebetween to reduce aggressive cutting into the canal sidewalls.
Typical prior art files are described in U.S. Pat. No. 4,934,934, U.S. Pat. No. 5,628,674 and U.S. Pat. No. 5,653,590, which patents are herewith incorporated by reference.
Since root canals are seldom straight, often having bends and twists, the endo files must be flexible. Thus, materials of construction have evolved from stainless steel to nickel-titanium alloys that have enhanced flexibility yet are further characterized as being strong and not likely to fail in use. It is obviously essential to a successful root canal procedure to avoid file breakage during the cleaning process.
The files may be designed to be manually manipulated or to be "engine driven", that is, fitted to a handpiece that provides rotation of the file during its use. An endodontic file that is intended for hand use is typically provided with an enlarged diameter plastic handle attached to the file metal shaft, configured for easy manipulation between the thumb and forefinger of the practitioner. A file intended for use with a handpiece has a shaft portion or stem at the file proximal end configured to be removably received within a chuck of the handpiece, by which the file may then be rotated as desired by a practitioner.
It has recently become apparent that a root canal can be more successfully cleaned, shaped and filled if the completed canal tapers into a conical shape from the coronal area of the tooth towards the tooth apex, the canal cleaning and shaping procedure being advanced from the crown down to the apex. That is, it is easier to achieve complete cleaning and shaping and successful filling of a canal if the canal is tapered in a generally conical configuration rather than being of substantially no taper or approximating natural taper from the top to the apex. For this reason, a preferred endodontic file is tapered over its active or working portion, the tapering extending from its maximum diameter adjacent the proximal end to a minimum diameter at the file distal end. While conventional files were once limited to a standard taper of 0.02 mm/mm of working length of 16 mm, files are now often preferred to be of a greater taper, such as 0.04, 0.05, 0.06, which greatly aids in preparing the canal "crown-down" to achieve an optimal conical canal shape.
In current endodontic practice, a series of files of increasing diameters and/or tapers are used as the practitioner gradually cleans and shapes a root canal. The practitioner selects files with the goal of achieving the most optimal configuration of a prepared root canal, that is, a canal conically tapered from the tooth coronal area to a smallest diameter at the tooth apex.
In preparing the root canal, after opening and cleaning the crown and its adjacent pulp chamber, the practitioner often first selects a file having a greater taper than 0.02 mm/mm of length. In the engine driven procedure the file is rotated at low speed and advanced into the root canal opening. The file working surfaces cut, scrap or plane canal walls, first engaging only a limited portion of the canal walls. The scraping surfaces become more and more engaged as the file advances and the canal surfaces conform to the conical shape of the file. More and more torque must be applied to the file to overcome increasing frictional forces and to turn the file at desired speed as the file advances. Typically, the practitioner advances the file until the resulting frictional forces have substantially reduced the ease of further advancement. Further advancement raises the danger of stalling or locking-up the file. In such event, torque applied to the file may suddenly increase and the file could fail and break, leaving the practitioner with an extremely difficult extraction.
It would be an improvement in the art of endodontics to provide a file that has a reduced tendency to stall or lock-up when engaged in cleaning a root canal. Such a file would have a reduced torque input capability that limits torque that can be applied to the file such that the risk of a catastrophic failure is reduced or eliminated.